In recent years, as alloys for rare-earth magnets, there have been R-T-B type alloys with excellent magnetic properties. Here, “R” in the “R-T-B type alloys” represents rare-earth elements, “T,” transition metals in which Fe is indispensable, and “B,” boron.
Alloy flakes of the R-T-B type alloys can be produced by the following steps.
(a) An ingot in a thin strip shape with a thickness of 0.01 to 2 mm is cast from a molten R-T-B type alloy through a strip casting method or the like.
(b) The cast thin-strip-shaped ingot is crushed into alloy flakes.
(c) The alloy flakes are cooled.
Here, in order to prevent oxidation of R-T-B type alloys, the above steps (a) to (c) are usually performed under reduced pressure or under an inert gas atmosphere.
Casting through the strip casting method can be performed, for example, with the following steps.
(A) Raw materials are charged into a crucible and heated, thereby being melted into a molten R-T-B type alloy.
(B) This molten alloy is poured via a tundish onto a copper roll having a structure in which a coolant circulates.
(C) The molten alloy poured onto the copper roll is rapidly cooled and solidified, so that an ingot in a thin strip shape is cast.
The alloy flakes formed by the R-T-B type alloy have an alloy crystal structure in which a crystallinity phase (main phase) formed by an R2T14B phase and an R-rich phase in which the rare earth element is concentrated coexist. The main phase is a ferromagnetic phase contributing to a magnetizing effect, and the R-rich phase is a non-magnetic phase not contributing to a magnetizing effect. The alloy crystal structure including the main phase and the R-rich phase can be evaluated using the crystal grain size of the main phase (hereinafter, referred to as a “main phase grain size”) in a cross-section of the alloy flake cut in the thickness direction (a cross-section in the thickness direction). The main phase grain size in an alloy flake obtained by crushing an ingot cast by the strip casting method is typically 3 to 5 μm.
Meanwhile, the main phase grain size of alloy flakes can be coarsened by subjecting the alloy flakes to heat treatment of heating to and holding at a predetermined temperature for a predetermined time and then cooling under reduced pressure or under an inert gas atmosphere. Specifically, alloy flakes produced by the above steps (a) to (c) are subjected to a heat treatment in which the alloy flakes are heated and held for a predetermined time and then cooled, so that the main phase grain size can be coarsened.
Alternatively, the main phase grain size of alloy flakes can be coarsened by replacing the cooling (rapid cooling) treatment in (c) in the production of alloy flakes through the above-described steps (a) to (c) with a heat treatment in which the alloy flakes are heated to and held at a predetermined temperature for a predetermined time and then cooled. Specifically, high-temperature alloy flakes immediately after being made by crushing a cast ingot are subjected to heat treatment without being cooled, whereby the main phase grain size of the alloy flakes can be coarsened. The series of heat treatments in which high-temperature alloy flakes immediately after being made by crushing an ingot are heated to and held at a predetermined temperature for a predetermined time and then cooled is also referred to as a “slow cooling treatment,” hereinafter.
To produce alloy flakes formed by R-T-B type alloys, various proposals have been made as shown in Patent Literatures 1 and 2, for example. An alloy flake production system described in Patent Literature 1 is configured by a melting and casting chamber in which alloy flakes are obtained by casting and the alloy flakes are placed on a conveyer, a heat treatment chamber in which the alloy flakes placed on the conveyer are heated while being transported, and a cooling chamber in which the alloy flakes are rapidly cooled to be discharged to atmospheric pressure. In the alloy flake production system described in Patent Literature 1, the melting and casting chamber, the heat treatment chamber, and the cooling chamber are connected through partitioning doors, thus allowing alloy flakes placed on the conveyer to be sequentially processed in a batch mode without being exposed to the atmosphere.
In an alloy flake production system described in Patent Literature 2, alloy flakes made by casting are dropped onto a dish-shaped container rotating at a low speed to be subjected to annealing. The alloy flakes dropped on the rotating dish-shaped container are allowed to spread over the entire surface of the dish-shaped container and stirred by a plurality of plowshares that is pressed against the surface of the dish-shaped container. In this way, the alloy flake production system described in Patent Literature 2 allows uniform heating treatment on the alloy flakes.